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SubjectRe: [PATCH] Fix compat_sys_sigpending breakage introduced by v4.13-rc1~6^2~12
On Sat, Aug 05, 2017 at 11:00:50PM +0300, Dmitry V. Levin wrote:
> The latest change of compat_sys_sigpending has broken it in two ways.
>
> First, it tries to write 4 bytes more than userspace expects:
> sizeof(old_sigset_t) == sizeof(long) == 8 instead of
> sizeof(compat_old_sigset_t) == sizeof(u32) == 4.
>
> Second, on big endian architectures these bytes are being written
> in the wrong order.

> @@ -3303,12 +3303,15 @@ SYSCALL_DEFINE1(sigpending, old_sigset_t __user *, set)
> #ifdef CONFIG_COMPAT
> COMPAT_SYSCALL_DEFINE1(sigpending, compat_old_sigset_t __user *, set32)
> {
> +#ifdef __BIG_ENDIAN
> sigset_t set;
> - int err = do_sigpending(&set, sizeof(old_sigset_t));
> - if (err == 0)
> - if (copy_to_user(set32, &set, sizeof(old_sigset_t)))
> - err = -EFAULT;
> + int err = do_sigpending(&set, sizeof(set.sig[0]));
> + if (!err)
> + err = put_user(set.sig[0], set32);
> return err;
> +#else
> + return sys_rt_sigpending((sigset_t __user *)set32, sizeof(*set32));
> +#endif

Interesting... Basically, your fix makes it parallel to compat rt_sigpending(2);
I agree that the bug is real and gets fixed by that, but... rt_sigpending()
itself looks a bit fishy. There we have
compat_sigset_t set32;
sigset_to_compat(&set32, &set);
/* we can get here only if sigsetsize <= sizeof(set) */
if (copy_to_user(uset, &set32, sigsetsize))
err = -EFAULT;
in big-endian case; now, there are 4 callers of sigset_to_compat() in the
entire kernel. One in sparc compat rt_sigaction(2), the rest in kernel/signal.c
itself. All are followed by copy_to_user(), and all but the sparc one are
under that kind of "if it's big-endian..." ifdefs.

Looks like it might make sense to do this:
put_compat_sigset(compat_sigset_t __user *compat, const sigset_t *set, int size)
{
#ifdef
compat_sigset_t v;
switch (_NSIG_WORDS) {
case 4: v.sig[7] = (set->sig[3] >> 32); v.sig[6] = set->sig[3];
case 3: v.sig[5] = (set->sig[2] >> 32); v.sig[4] = set->sig[2];
case 2: v.sig[3] = (set->sig[1] >> 32); v.sig[2] = set->sig[1];
case 1: v.sig[1] = (set->sig[0] >> 32); v.sig[0] = set->sig[0];
}
return copy_to_user(compat, &v, size) ? -EFAULT : 0;
#else
return copy_to_user(compat, set, size) ? -EFAULT : 0;
#endif
}

int put_compat_old_sigset(compat_old_sigset_t __user *compat, const sigset_t *set)
{
/* we want bits 0--31 of the bitmap */
return put_user(compat, set->sig[0]);
}

in kernel/signal.c and turn e.g. compat rt_sigpending() into
COMPAT_SYSCALL_DEFINE2(rt_sigpending, compat_sigset_t __user *, uset,
compat_size_t, sigsetsize)
{
sigset_t set;
int err = do_sigpending(&set, sigsetsize);
if (!err)
err = put_compat_sigset(uset, &set, sigsetsize);
return err;
}

etc., including
COMPAT_SYSCALL_DEFINE2(sigpending, compat_old_sigset_t __user *, uset)
{
sigset_t set;
int err = do_sigpending(&set, sizeof(set));
if (!err)
err = put_compat_old_sigset(uset, &set);
return err;
}

Incidentally, this
if (sigsetsize > sizeof(sigset_t))
return -EINVAL;
should probably be lifted out of do_sigpending() into rt_sigpending() and its
compat analog - sigsetsize argument is not used anywhere else in do_sigpending(),
and rt_sigpending() (and *especially* its compat counterpart) would be a lot
more obviously correct with that check done where it's seen.

Anyway, all of that can be done later; for now let's go with your patch.
ACKed-by: Al Viro <viro@zeniv.linux.org.uk>

I would pick it through my tree, but the local network is half-disasembled
for move (containers arrive tomorrow, flight to Boston on 9th, stuff should
arrive there by the weekend, so I hope to be back to normal by the 14th
or so, assuming I'll have any sanity left by that time). Could somebody
else pick that one? Linus?

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